A familiar stress bearing support structure for passenger vehicles in the form of a lattice-frame made with hollow section beams is disclosed in European patent EP 0 146 716. The hollow section beams, formed as extruded aluminum sections, are held together by aluminum joint elements made by casting or forging. Such a "transfer joint technique" utilizing the cast aluminum joint elements provides a sturdy stress bearing structure with strong connections between the aluminum extrusions. However, the above-described transfer joint technique is also, on the whole, cost-intensive.
Butt welding is a simple and inexpensive technique for joining aluminum extrusions together at right angles. It is well known that thermal welding of heat treatable aluminum alloys produces a significant decrease in the strength of material in the so-called "zone of thermal influence" immediately adjacent the weld zone. For this reason, a welded butt-joint connection has a much lower stress carrying capability than a transfer joint made with cast aluminum joint elements. Thus, simple welded butt-joint connections are not suitable for many high stress joint regions of a passenger vehicle framework assembled from extruded aluminum frame elements. In particular, it is desirable to avoid use of transverse weld connections on stress bearing hollow section longitudinal members made of extruded light metal.
In addition, other connection techniques and arrangements for joining light alloy, hollow section frame members of superstructure frameworks for street and rail vehicles are known from DE 21 28 281 B2 and DE 27 51 753 C2. Here, a number of vertically and horizontally oriented hollow section bars are arranged at right angles to each other to form the framework of a wall. Triangular corner brackets are placed in the angles at the joints and are welded in place to provide reinforcement. In this arrangement, the framework is stabilized, especially against shearing movements of the hollow-section bars in the wall plane. On the other hand, lateral stresses on the junction points of the bars are absorbed and stayed only to a small degree by such corner brackets. Such connections are therefore not well suited for junction points of structural members in passenger vehicles which must absorb and stay high levels of lateral stress, such as occurs in a collision event.
One junction point in a motor vehicle body that must be able to withstand a high level of lateral stress is the upper rear corner joint region of the vehicle roof formed by the three way junction of the longitudinal roof girder, the B column, and the transverse roof girder. This particular junction point must be constructed so that it is capable of sustaining great stress in order to prevent the vehicle roof from collapsing during a roll over.